1. Field of the Invention
The present invention relates to an illumination system for a projection exposure system having a light source, the light source having a source size Q and emitting radiant power in a spatial angle element Ω(α), at least one collector, which receives or picks up radiant power from a source up to a first, maximum aperture angle αmax(1), a first geometric flux G1 being defined by the source size Q and the first maximum aperture angle αmax(1), as well as a plane to be illuminated, which is preferably a reticle plane, in which for example a mask is positioned. The reticle plane has an area A and a numeric aperture NA to be illuminated, the numeric aperture NA defining a second aperture angle α(2) and a second geometric flux being determined by the area A and the second aperture angle α(2).
2. Description of the Related Art
Projection exposure systems for microlithography, particularly for wavelengths ≦193 nm, have become known from multiple applications. Reference is made to German Patent Application 100 20 592 and its counterpart, U.S. Pat. No. 6,424,471 in regard to catadioptric systems and to German Patent 198 55 157 and its US-counterpart U.S. Pat. No. 6,522,484 in regard to refractive systems, the content of whose disclosures are included in their entirety in the present application.
Currently, wavelengths in the range 11–14 nm, particularly 13.5 nm, are discussed as wavelengths for EUV lithography at a numerical aperture of 0.2–0.3. The image quality in EUV lithography is determined by the projection objective and by the illumination system. The illumination system should provide an uniform illumination as far as possible of a field plane, in which the structure-bearing mask, the reticle, is positioned. The projection objective images the field plane in an image plane, the wafer plane, in which a light-sensitive object is positioned. Projection exposure systems for EUV lithography are implemented using reflective optical elements. The shape of the field in the image plane of an EUV projection exposure system is typically that of an annular field having a high aspect ratio of 2 mm (width)×22–26 mm (arc length). The projection systems are typically operated in scanning mode. Reference is made to the following publications in regard to EUV projection exposure facilities:
W. Ulrich, S. Beiersdörfer, H. J. Mann, “Trends in Optical Design of Projection Lenses for UV- and EUV-Lithography” in Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Ed.), Proceedings of SPIE, Vol. 4146 (2000), pp. 13–24, and
M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B. Kruizinga, “Illumination Optics Design for EUV-Lithography” in Soft X Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Ed.), Proceedings of SPIE, Vol. 4146 (2000), pp. 25–34,
the content of these disclosures are included in their entirety in the present application.
Special illumination systems for EUV projection exposure systems have become known from the following cited publications. Thus, U.S. Pat. No. 5,339,346 discloses an illumination system for a lithography device which uses EUV radiation. For uniform illumination in the reticle plane and filling of the pupil, U.S. Pat. No. 5,339,346 suggests a condenser, which is constructed as a collector lens and includes at least four paired mirror facets, which are positioned symmetrically. A laser plasma light source is used as a light source.
An illumination system having a laser plasma light source, including a condenser mirror, with which illumination of a mask and/or reticle to be illuminated is achieved with the aid of spherical mirrors, is disclosed in U.S. Pat. No. 5,737,137.
U.S. Pat. No. 5,361,292 discloses an illumination system, in which a light source is provided and the point like source is imaged into a ring shaped surface to be illuminated with the aid of a condenser, which has five aspheric mirrors, positioned off-center. The ring shaped illuminated surface is then imaged in the entrance pupil with the aid of a special sequence of grazing-incident mirrors downstream.
An illumination system, in which a photon radiation source is split into multiple secondary light sources with the aid of a honeycomb condenser, is known from U.S. Pat. No. 5,581,605. In this way, uniform illumination in the reticle plane is achieved. The reticle is imaged on the wafer to be exposed with the aid of a typical reduction optic. Precisely one mirror with raster elements having identically curved raster elements as honeycomb condensor is provided in the illumination beam path. In case of U.S. Pat. No. 5,581,605, a laser plasma source or a very small source is assumed.
European Patent Application EP-A-0 939 341 and its US counterpart U.S. Pat. No. 6,452,661 discloses a Koehler illumination system for wavelengths <200 nm, particularly for the EUV range, having a first optical integrator comprising a plurality of first raster elements and a second optical integrator comprising a plurality of second raster elements. A laser plasma source is described as a light source.
A further EUV illumination system, which comprises two mirrors or lenses having raster elements, is known from German Patent Application DE 199 03 807 A1 and its US-counterpart U.S. Pat. No. 6,438,199. These types of systems are also referred to as double faceted EUV illumination systems. The content of the disclosure of this application is included in its entirety in the present application.
The principle of the construction of a double faceted EUV illumination system is disclosed in German Patent Application DE 199 03 807 A1 and its US-counterpart U.S. Pat. No. 6,438,199. The illumination in the exit pupil of the illumination system according to German Patent Application DE 199 03 807 and its US-counterpart U.S. Pat. No. 6,438,199 is determined by the arrangement of the raster elements on the second mirror. A small laser plasma source or a pinch plasma source having a small angle of radiation is again described as a light source.
The content of the disclosures of all applications are included in their entirety in the present application.
Illumination systems as described above have either a laser plasma source having a small dimension or a pinch plasma source, which only radiates in a small spatial angle, as a light source. In both cases, the geometric flux which may be picked up by the collector of an illumination system is therefore limited.
If larger plasma sources, such as discharge plasma sources, are used instead of the light sources described before, too much light is picked up, which cannot be used in the reticle plane to illuminate the structure-bearing mask. If too much light is collected by the collector, an undesired high thermal load arises in the subsequent illumination system and the light collected in excess must be blocked using e.g. diaphragms. On the other hand, if too little light is collected, in a collector which is tailored to the pinch plasma source, for example, too little light is made available to the light-sensitive object in the wafer plane. Furthermore, the high requirements for homogenity of the illumination in the reticle plane cannot be fulfilled.